Conventionally, a power supply apparatus that reduces a direct current supplied from a direct current power source (e.g., a battery) to a predetermined voltage is classified into two types; one type using a voltage regulator and the other type using a DC-to-DC converter.
FIG. 1 shows an exemplary circuit of a background power supply apparatus using a voltage regulator 100. In the voltage regulator 100 of FIG. 1, a P-channel-type MOS (metal oxide semiconductor) transistor 102 (hereinafter referred to as a P-MOS transistor 102) and resisters 103 and 104 are connected in series between a power source terminal applied with a power source voltage VDD by a direct current 101 (e.g., a battery including a secondary battery) and ground. The resisters 103 and 104 divide a voltage Vout which is compared by a voltage comparator 106 with a predetermined reference voltage Vref generated by a reference voltage generator 105. Based on a comparison result, an operation of the P-MOS transistor 102 is controlled so that the voltage Vout is held at a desired value. In FIG. 1, a CPU 107 is an exemplary system that requires power from the voltage regulator 100.
However, the above-described voltage regulator has a drawback that the P-MOS transistor 101 consumes a great amount of electric power for a reduction of the power source voltage VDD to the voltage Vout. More specifically, when the CPU 107 consumes a current of 100 mA, for example, and a voltage regulator 100 reduces the power source voltage VDD from 3.6 volts, for example, to 2 volts, for example, the P-MOS transistor 101 consumes the power of 0.16 W. That is, the voltage regulator consumes a difference of the battery voltage and the CPU's operational voltage. Such voltage regulator is undesirable for a system aiming a low power consumption since the CPU's operational voltage has been lowered in the recent years.
Accordingly, as shown in FIG. 2, a DC-to-DC converter is used in place of the voltage regulator as a power supply in a system (e.g., the CPU 107) using a battery. In FIG. 2, a DC-to-DC converter 110 reduces the power source voltage VDD to a predetermined voltage Vout and supplies the voltage Vout to the CPU 107.
In general, a system using a battery as a source of power is provided with a sleep function or temporarily stopping the operations of the system to reduce an electrical power consumption on an as needed basis. In the case of the power supply apparatus of FIG. 2, it is attempted to reduce the power consumption by changing the output terminal of the DC-to-DC converter 110 to the CPU 107 in the sleep mode from a ground level to a high impedance level. This is because the DC-to-DC converter 110 is used as an apparatus that directly controls the power source required by the system (e.g., the CPU 107).
On the other hands, the DC-to-DC converter 110 is required to be always in an active state in the case the system (e.g., the CPU 107) in the sleep mode is intermittently activated to control certain components on an as needed basis. In such a case, the power consumption by the DC-to-DC converter 110 shares a large part of the total system power consumption.